Cables between Bletchley Park and Washington, D.C. during WWII, exchanging solutions of Enigma cipher machines used by Nazi spies in South America. The Bletchley Park section that solved the spy Enigmas was known as ISK, Intelligence Service Knox, and the American section was the Coast Guard Cryptanalytic Unit (a.k.a. Unit 387), created by Elizebeth Smith Friedman. The two sections worked independently and ended up solving the machines around the same time.
National Archives UK location: HW 19/361.